Current satellite systems provide positioning and time information by broadcasting navigation signals to properly equipped users. For example, a the US Global Positioning System (GPS) consists of 24 satellites orbiting the earth twice a day at an altitude of approximately twelve thousand miles, as well as a network of ground stations to monitor and manage the satellite constellation. The GPS satellites transmit continuous Navigation Data and Ranging (NDR) information 24 hours a day toward the earth. A GPS receiver which properly decodes, tracks and interprets these transmissions from the GPS satellites can compute the position of the GPS receiver as well as determine accurate time. The basic functioning of GPS and GPS receivers, is well known in the art. The GPS satellite system currently broadcasts for civilian use a Standard Positioning Service (SPS) on a single frequency (1575.42 MHz) called L1. The current GPS receivers and the GPS satellites are not capable of two-way communication with each other. GPS is a broadcast only service.
The GPS was conceived, designed and deployed as a military force enhancement. Consequently much of the capability of the GPS (i.e. the Precise Positioning Service or PPS) is not available to Civil users. Furthermore, even the GPS SPS service which is available to the civil community was not designed with adequate integrity, reliability or availability necessary to support safety of life civil applications. Furthermore, the SPS includes a relatively low power signal on only a single frequency and is consequently vulnerable to intentional or unintentional interference. These problems with the integrity and robustness of the civil GPS services are well known in the art.
As the SPS signals travel from the GPS satellites to the GPS receivers the SPS signals travel through the ionosphere which encircles the earth. The ionosphere acts as a dispersive medium and refracts the SPS signals as they travel through the ionosphere. As a result, the SPS signals do not appear to travel at the speed of light, which is assumed in the calculation of the position of the GPS receiver. The ionospheric induced delay in the reception of the SPS signals limits the accuracy of the determination of the position of the GPS receiver and is the largest location dependent error source in the calculation of the position of the GPS receiver. Therefore, the use of the GPS SPS signals to compute a position of the GPS receiver has limited accuracy and cannot be used for applications requiring a high degree of precision in the determination of the position of the GPS receiver.
To overcome the aforementioned shortcomings of the GPS, a number of space based augmentation systems (SBAS) are under development. For example, there are currently three SBAS systems under development world-wide: the Wide Area Augmentation System (WAAS) under development by the Federal Aviation Administration; the European Geostationary Navigation Overlay Service (EGNOS) under development by the European Space Agency in conjunction with EURO CONTROL and the European Union; and the MTSAT Satellite Augmentation System (MSAS) under development by the Japanese Civil Aviation Bureau. These SBASs provide for a way to measure and correct for the ionospheric delay caused by the SPS signals traveling through the ionosphere on its way toward earth and provide for basic integrity monitoring of the GPS SPS service sufficient to meet the requirements for civil aviation applications. However, all these SBAS will operate on the same GPS L1 frequency and will ultimately depend on the availability of basic GPS SPS. Hence SBAS does little or nothing to address the robustness concerns of GPS.
The electron density of the ionosphere varies as a function of geographic location. In a vectorized, wide area differential solution such as that employed by an SBAS, a large number of sampling locations are needed to compute an accurate model of the variation of the time delay induced in a signal traveling through various locations in the ionosphere. Therefore, in order to get adequate sampling of the state of the ionosphere, the SBASs employ a number of reference stations over a wide region that are fixed to the earth. These reference stations are connected via a ground based telecommunications network to a central processing facility. Each reference station observes the transmitted SPS signals from the GPS satellites visible at the reference station, performs some signal integrity monitoring, and passes the data on to the central processing facility via the ground based telecommunications network. These stations also track a component of the PPS using a codeless tracking technique in order to make dual frequency measurements of the ionosphere. The central processing facility uses the data from the reference stations to compute “wide-area” differential corrections where separate corrections are given for various satellite pseudo range error components. The SBASs then provide estimates of the vertical ionospheric delay at predefined grid points over the region covered by the SBAS to users of the SBAS. The estimates are broadcast from the SBAS to the user via a satellite link which is designed to be very similar to a GPS signal. The GPS receiver can then compute an estimate of the ionospheric delay for each pseudo range based on the user's location and the geometry of the satellites and compute its position more accurately by accounting for the ionospheric delay in the SPS signals and by applying the other differential correction components included in the SBAS signal.
The SBAS architecture is attractive in that it supports operations over a wide area and may even be capable of providing a level of service sufficient to support category 1 precision approach aircraft operations. However, the complexity and cost of such a system makes it impractical for most States or regions to consider employing such a system. Particularly, the cost of the ground based telecommunications network can be very significant. Also, in order to get good sampling of the ionosphere and a more accurate grid of the errors introduced by the ionosphere, a large number of reference stations are required, which in turn increases the cost of connecting all the reference stations with the ground based telecommunication networks.
Therefore, it is desirable to develop a system and method for accurately measuring and correcting the time delay induced in signals traveling through the ionosphere. Additionally, it is desirable to perform the ionospheric delay sampling without the need for an extensive network of ground based monitoring stations. It is also desirable to provide monitoring stations without the need for the monitoring stations to be connected to the central processing facility by expensive ground based telecommunication networks.